The present invention relates to plasma arc torches.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treating, melting, and annealing. Such torches include an electrode which supports an electric arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. In some types of torches, a second shielding gas is used to surround the jet of plasma gas and the arc for controlling the work operation. In other types of torches, a swirling jet of water is used to surround the jet of plasma gas and the arc and impinge on the workpiece for controlling the work operation.
One characteristic of existing plasma arc torches is that there is little or no commonality between shielding gas torches and water-injection torches. Thus, a user who desires to employ both gas-shielded and water-injected plasma arc processes must purchase two complete torch assemblies. Furthermore, a plasma arc torch manufacturer who desires to make both types of torches must manufacture and maintain inventories of two complete sets of different components, and therefore the cost complexity of the manufacturing operation are increased.
In a typical plasma arc torch, the plasma gas and the shielding gas or water are directed by a nozzle assembly having a plasma gas nozzle and a shielding gas or water injection nozzle coaxially arranged concentrically or in series. The nozzle assembly is electrically conductive and is insulated from the electrode so that an electrical potential difference can be established between the electrode and the nozzle assembly for starting the torch. To start the torch, one side of an electrical potential source, typically the cathode side, is connected to the electrode and the other side, typically the anode side, is connected to the nozzle assembly through a switch and a resistor. The anode side is also connected in parallel to the workpiece with no resistor interposed therebetween. A high voltage and high frequency are imposed across the electrode and nozzle assembly, causing an electric arc to be established across a gap therebetween adjacent the plasma gas nozzle discharge. This arc, commonly referred to as a pilot or starting arc, is at a high frequency and high voltage but a relatively low current to avoid damaging the torch. Plasma gas is caused to flow through the plasma gas nozzle to blow the pilot arc outward through the nozzle discharge until the arc attaches to the workpiece. The switch connecting the potential source to the nozzle assembly is then opened, and the torch is in the transferred arc mode for performing a work operation on the workpiece. The power supplied to the torch is increased in the transferred arc mode to create a cutting arc which is of a higher current (and typically a lower voltage) than the pilot arc.
Because of the relatively high voltages and currents used in such torches, the electrode and nozzle assembly become hot and must be cooled to prevent early failure of the torch. Accordingly, high-current plasma arc torches generally include coolant circuits for flowing a coolant around the nozzle assembly and/or the electrode. The liquid coolants used often are capable of conducting electricity to some extent. In water-injection torches, unless deionized water is used for the injection water, the injection water is also capable of conducting electricity to some extent. In addition, some shielding gases are conductive, such as argon.
One of the problems with some existing plasma arc torches is current leakage between the electrode potential and the nozzle potential caused by injection water, shielding gases, and/or coolant flowing between adjoining surfaces of various parts of the torches and making its way from a part at electrode potential to a part at nozzle potential. When this happens, a larger voltage potential must be imposed across the electrode and nozzle assembly in order to establish the starting arc. If the current leakage is severe enough, starting the torch can be difficult or nearly impossible with reasonably manageable levels of voltage.
Another problem with some existing torches is that the shield gas or injection water typically flows through a component of the torch which is at electrode potential and then comes into contact with a component of the torch at nozzle potential over a path of relatively short length. Depending on the shielding gas or the type of injection water used, it is possible for current to leak via this path through the shielding gas or injection water. Thus, even if adequate precautions are taken to seal connections between parts to prevent wetting of adjoining component surfaces, there is still a potential leakage path which can make starting the torch difficult.
A further disadvantage of some existing torches is that the electrical conductor wire which is connected to the nozzle assembly is routed internally through the torch and is secured by a set screw in a hole in a contact ring with which the nozzle assembly makes contact when the torch is assembled. The contact ring must frequently be removed and replaced to enable replacement of certain parts that wear out. When replacing the contact ring, it can be difficult to engage the end of the conductor wire in the hole in the contact ring, especially if the end of the wire is frayed or bent. Moreover, the wire can become pushed back into the torch if there is interference between the contact ring hole and the wire.
In summary, existing plasma arc torches are subject to several disadvantages, namely, lack of commonality between gas-shielded and water-injection torches, current leakage through various leakage paths, and difficulty making an electrical connection between the nozzle and the conductor leading to the power supply when assembling the torch.
The present invention enables commonality between gas-shielded and water-injection torches so that a user or manufacturer can assemble either type of torch by starting with a common torch body. Accordingly, the user who performs both gas-shielded and water-injection processes is afforded greater flexibility in adapting a plasma arc torch system to the needs of a particular process, and can perform both types of processes with a smaller total capital investment in equipment. Furthermore, a manufacturer of both gas-shielded and water-injection torches potentially can achieve greater manufacturing efficiencies on the parts in common between the two types of torches.
To these ends, the invention in accordance with a first embodiment thereof provides a plasma arc torch assembly which includes a main torch body having first and second end faces, and a control fluid passage and a plasma gas passage each extending from the first end face through the main torch body and out the second end face. The control fluid passage is adapted to carry either a shielding gas or injection water. The torch assembly further comprises one of a shielding gas insulator body and a water-injection insulator body. The shielding gas insulator body has opposite first and second ends and a shielding gas passage and a plasma gas passage each extending from the first end through the insulator body, the first end of the insulator body being structured to be received against the second end of the main torch body, and the shielding gas and plasma gas passages being alignable with the control fluid and plasma gas passages, respectively, of the main torch body. Similarly, the water-injection insulator body has opposite first and second ends and an injection water passage and a plasma gas passage each extending from the first end through the water-injection insulator body, the first end of the water-injection insulator body also being structured to be received against the second end of the main torch body, and the injection water and plasma gas passages being alignable with the control fluid and plasma gas passages, respectively, of the main torch body.
The torch assembly also includes a nozzle adapted to be juxtaposed with the second end of either of the insulator bodies for receiving plasma gas from the plasma gas passage thereof, and an electrode having a discharge end adapted to be juxtaposed with the nozzle and to support an electric arc extending therefrom through the nozzle to a workpiece.
The main torch body is assemblable with either the shielding gas insulator body or the water-injection insulator body such that plasma gas is passed from the main torch body through the insulator body and to the nozzle and one of the shielding gas and water is passed through the respective insulator body to surround the arc. The torch assembly of the invention thus enables a user to assemble either a gas-shielded torch or a water-injection torch by starting with a common main torch body.
The invention also overcomes the other disadvantages of existing torches noted above by providing a plasma arc torch having novel sealing connections between the fluid passages of adjoining parts of the torch such that wetting of adjoining surfaces is substantially reduced. In another aspect of the invention, a plasma arc torch is provided having a novel connection for supplying shielding gas or injection water to the nozzle assembly of the torch such that the electrical path through the shielding gas or injection water is substantially lengthened relative to existing torches, thus substantially reducing the likelihood of significant current leakage during starting. In yet another aspect of the invention, a plasma arc torch is provided having a novel electrical connector assembly for connecting the electrical conductor wire to a contact member of the torch such that the wire is held in a position permitting the contact member to be connected with the wire.
To these ends, a plasma arc torch in accordance with a further preferred embodiment of the invention comprises a main torch body having a fluid passage extending through the body and through an end face of the body for passing a fluid such as plasma gas, shielding gas, or injection water to an electrode and/or nozzle assembly of the torch. An insulator body is connected to the main torch body with an end face of the insulator body confronting the end face of the main torch body. The insulator body includes a fluid passage which extends therethrough and through the end face of the insulator body in alignment with the fluid passage of the main torch body. A connector assembly fluidly couples the fluid passages and preferably comprises a coupling tube having a first portion sealingly received within the fluid passage of the main torch body and a second portion sealingly received within the fluid passage of the insulator body.
Preferably, the coupling tube includes resilient compressible seals encircling the first and second portions of the coupling tube. The seals are compressed between the tube and the inner surfaces of the fluid passages to prevent fluid from flowing between the tube and the inner surfaces of the passages. In a particularly preferred embodiment of the invention, each seal comprises an O-ring, and more preferably a pair of O-rings spaced apart lengthwise along the coupling tube, the space between the O-rings establishing an insulating air space.
The connector assembly thus substantially reduces the likelihood of fluid making its way between the confronting end faces of the main torch body and insulator body and establishing an electrical path from the main torch body to another part of the torch at nozzle potential. Furthermore, when the torch is disassembled and the insulator body is disconnected from the main torch body, the connector assembly reduces the likelihood that residual fluid residing in the adjoining fluid passages will come in contact with the adjoining end faces or other surfaces of the bodies. Potential current leakage paths are thus reduced significantly.
In accordance with another aspect of the invention, a plasma arc torch includes an electrically conductive main torch body which has a control fluid passage extending through the body to an end face at an end of the body. The control fluid passage is for supplying a control fluid such as shielding gas or injection water to the torch. The connector tube advantageously is adapted to be connected to a control fluid supply hose with a coupling. A torch end assembly is connected to the main torch body and includes an insulator body having a control fluid passage which extends through an end face which confronts the end face of the main torch body for receiving control fluid from the main torch body. The torch end assembly also includes a nozzle assembly having a control fluid nozzle which receives control fluid from the passages of the main torch body and insulator body.
In order to lengthen the electrical path from the main torch body through the control fluid to the torch end assembly, an elongate electrically insulating conduit is disposed in the control fluid passage of the insulator body and extends through the control fluid passage of the main torch body. The insulating conduit has a first portion that forms a seal with the passage of the insulator body and a second portion that forms a seal with the passage of the main torch body in order to prevent control fluid from establishing an electrical path between the conduit and the control fluid passages of the main torch and insulator bodies. Thus, the electrical path from the main torch body to the torch end assembly extends from the end of the insulating conduit through the control fluid passages of the main torch and insulator bodies. The total resistivity of the path is thereby increased substantially, making current leakage less likely during starting of the torch.
The insulating conduit preferably is sealed by resilient compressible seals which are compressed between the conduit and the inner surfaces of the control fluid passages. The seals preferably comprise O-rings retained in grooves formed in the outer surface of the conduit. More preferably, each seal comprises a pair of O-rings spaced apart along the length of the conduit and retained in a pair of spaced-apart grooves in the conduit. An insulating air space is established between the two O-rings of each seal.
In addition to lengthening the electrical path through the control fluid, the insulating conduit also improves the sealing of the fluid connection between the passages of the main torch body and insulator body so that wetting of adjoining surfaces is less likely when injection water is the control fluid. Thus, the insulating conduit also provides advantages similar to those of the connector assembly described above.
An insulator body in accordance with the invention comprises a solid body of electrically insulating material. The body has first and second opposite end faces, and an axial bore extending through the body from one end face to the other for receiving an electrode assembly of a torch. The insulator body has at least one fluid passage which originates at the first end face and extends through the insulator body for supplying a fluid to other components of a torch, such as a nozzle assembly. The fluid passage includes a receiving portion which originates at the first end face and is adapted to receive a fluid connector such as a coupling tube as described above. The receiving portion is generally cylindrical and includes a tapered or flared entrance portion adjacent the first end face of the insulator body to facilitate inserting a coupling tube into the receiving portion. The receiving portion preferably includes inner surfaces that define a stop for a fluid connector to abut when inserted into the receiving portion. The insulator body preferably includes a plurality of such fluid passages including a plasma gas passage and a control fluid passage, each passage having a receiving portion as described above.
The insulator body also includes a coolant supply passage extending from a first portion of the axial bore through the outer cylindrical surface of the insulator body for supplying a coolant to a plenum surrounding a nozzle assembly of a torch, and a coolant return passage extending from the outer cylindrical surface of the insulator body into a second portion of the axial bore between the first portion and the first end face of the insulator body for returning coolant through the axial bore to a coolant return passage of a main torch body.
In accordance with a further aspect of the invention, a plasma arc torch is provided having an improved means for connecting a conductor wire within the torch. The torch comprises a main torch body having a conductor passage extending through it and an electrical conductor disposed within the conductor passage with a free end of the conductor projecting from an outer end of the main torch body. An insulator body connects to the main torch body and includes a conductor receptacle for receiving the free end of the conductor. An access hole extends from an outer end of the insulator body into the receptacle. The free end of the conductor has an electrical connector assembly attached thereto, and the electrical connector assembly is received within the receptacle in the insulator body. An electrical contact member abuts the outer end of the insulator body, and is adapted to be contacted by a nozzle assembly of the torch. A fastener extends through the contact member and access hole and engages the electrical connector assembly for establishing electrical contact between the electrical connector assembly and the contact member. Thus, an electrical path extends through the conductor and electrical connector assembly to the contact member and thence to the nozzle assembly, which path is used during starting the torch to establish an arc between an electrode of the torch and the nozzle assembly.
To keep the electrical connector assembly from being pushed into the main torch body when assembling the insulator body and contact member onto the torch, an insulating sleeve surrounds the conductor and extends through the conductor passage of the main torch body and into the receptacle of the insulator body. The free end of the conductor extends out from the sleeve. The electrical connector assembly includes an electrical connector attached to the free end of the conductor and larger than the inner diameter of the sleeve, so that the electrical connector is prevented from being pushed into the sleeve. A collar is slidingly received over an end portion of the sleeve which projects out from the conductor passage of the main torch body, the collar being larger in diameter than the conductor passage, and a stop ring is affixed to the sleeve between the collar and the resilient compressible seal, the stop ring abutting the collar to prevent the seal from being withdrawn into the sleeve.
The electrical connector is thus held in a position projecting out from the main torch body so that the insulator body can be assembled to the main torch body and the contact member can be assembled to the insulator body without pushing the conductor into the torch.
The electrical connector preferably comprises a generally cylindrical connector having an axial hole therethrough for receiving the end of the conductor in one end of the hole. The other end of the connector is split. The fastener which secures the contact member to the insulator body includes an end portion that extends into the hole at the split end of the connector and spreads the split end apart.
The electrical connector preferably fits snugly into the receptacle in the insulator body so that the spreading apart of the split end of the connector is resisted by the inner walls of the receptacle, thus facilitating a good electrical connection between the connector and the fastener. To this end, the connector preferably includes a resilient compressible seal encircling the connector and adapted to be compressed between the connector and the inner surface of the receptacle. The seal preferably comprises a pair of O-rings spaced apart along the connector and retained in grooves in the connector.
The sleeve preferably includes a resilient compressible seal encircling the sleeve and adapted to be compressed between the sleeve and the inner surface of the receptacle in the insulator body for preventing liquid from establishing an electrical path from the main torch body through the receptacle and to the electrical connector. The seal preferably comprises a pair of O-rings spaced apart along the sleeve and retained in grooves in the sleeve.
A still further aspect of the invention provides a unique nozzle retaining cup assembly for retaining a nozzle assembly in a gas-shielded plasma arc torch, having a holder and a separately formed cup which is received and secured within the holder, wherein a shielding gas flow path is provided between the outer surface of the cup and the inner surface of the holder for supplying shielding gas to a shielding gas nozzle of the torch.
Additionally, the invention provides a unique electrode holder assembly including a tubular electrode holder and a coolant tube secured within the internal passage of the electrode holder, wherein the electrode holder includes a portion adapted to be received within a bore of an insulator body. The portion includes one or more holes through the side wall of the electrode holder for supplying coolant from the internal passage to a coolant supply passage in the insulator body. A seal adapted to seal against the inner wall of the bore in the insulator body is on the outer surface of the electrode holder located between the holes and the free end of the holder which is adapted to retain an electrode adjacent a nozzle assembly of a torch. A raised rib or dam on the outer surface of the holder is located on the opposite side of the holes from the seal and is adapted to cooperate with the inner wall of the bore in the insulator body to substantially prevent coolant flow past the dam. Coolant which has already cooled the nozzle assembly is returned through a coolant return passage in the insulator body into the bore at a location on the opposite side of the dam from the holes in the electrode holder. The dam discourages flow past the dam in the direction of the holes, so that returned coolant is routed through the bore for return to a coolant source outside the torch.
The invention in its various aspects thus provides a plasma arc torch having a number of significant advantages over prior torches, including the ability to readily convert a gas-shielded torch into a water-injection torch and vice versa, using one common torch body. The invention also promotes improved sealing of fluid connections within the torch so that potential current leakage paths are substantially reduced, and also provides a torch having features for lengthening the potential electrical path through a shielding gas or injection water flow path. The invention thus achieves the objective of reducing current leakage during starting, facilitating the starting of a torch more reliably and with lower voltage. The invention also provides a torch having an improved electrical connection means for establishing connection between an electrical potential source and a contact member of the torch, so that assembly of the torch is facilitated.